1. Field of Use
This invention relates to methods of forming gated filament cathode structures for a field emission display, and more particularly, to methods using particle tracking to define gate apertures, spacers as etch masks to form filaments between top planes and bottom planes of associated gate structures, and using the gate structures to define where the filament tip will be.
2. Description of the Related Art
Field emission displays include a faceplate, a backplate and connecting walls around the periphery of the faceplate and backplate, forming a sealed vacuum envelope. In some field emission displays, the envelope is held at vacuum pressure, which can be about 1×10−7 torr or less. The interior surface of the faceplate is coated with light emissive elements, such as phosphor or phosphor patterns, which define an active region of the display. Field emission cathodes, such as cones and filaments, are located adjacent to the backplate. Application of an appropriate voltage at the extraction electrode releases electrons which are accelerated toward the phosphors on the faceplate. The accelerated electrons strike their targeted phosphors, causing the phosphors to emit light seen by the viewer at the exterior of the faceplate. Emitted electrons for each of the sets of emitters are intended to strike only certain targeted phosphors.
Uniform illumination each phosphor is desirable if not necessary and requires, (i) a uniform phosphor, and (ii) provide a uniform number of electrons to each phosphor, thus requiring uniform electron (emitter) sources.
A variety of methods for forming field emitters are known.
U.S. Pat. No. 3,655,241 discloses fabricating field emitters using a screen with arrays of circular or square openings that is placed above a substrate electrode. A deposition is performed simultaneously from two sources. One of the sources consists of an emitter-forming metal, such as molybdenum, and atoms are deposited in a direction perpendicular to the substrate electrode. The other source consists of a closure material, such as a molybdenum-alumina composite. Atoms of the closure material are caused to impinge on the screen at a small angle to the substrate. The closure material progressively closes the openings in the screen. Thus the emitter-forming metal is deposited in the shape of cones or pyramids, depending on whether the screen openings are circular or square.
Another method of creating field emitters is disclosed in U.S. Pat. No. 5,164,632. Part of an aluminum plate is anodically oxidized to create a thin alumina layer having pores that extend nearly all the way through the alumina. An electrolytic technique is used to fill the pores with gold for the field emitters. An address line is formed over the filled pores along the alumina side of the structure, after which the remaining aluminum and part of the adjoining alumina are removed along the opposite side of the structure to re-expose the gold in the pores. Part of the re-exposed gold is removed during an ion-milling process utilized to sharpen the field emitters. Gold is then evaporatively deposited onto the alumina and partly into the pores to form the gate electrode.
Field emitters are fabricated in U.S. Pat. No. 5,150,192 by creating openings partway through a substrate by etching through a mask formed on the bottom of the substrate. Metal is deposited along the walls of the openings and along the lower substrate surface. A portion of the thickness of the substrate is removed along the upper surface. A gate electrode is then formed by a deposition/planarization procedure. Cavities are provided along the upper substrate surface after which the hollow metal portions in the openings are sharpened to complete the field emitter structures.
However, large area field emission displays require a relatively strong substrate for supporting the field emitters extending across the large emitter area. The requisite substrate thickness is typically several hundred microns to 10 mm or more.
The fabrication methods in U.S. Pat. Nos. 5,164,632 and 5,150,192 make it very difficult to attach the field emitters to the substrates of thickness required for large area displays.
In U.S. Pat. No. 4,940,916, a gated area field emitter consists of cones formed on a highly resistive layer that overlies a highly conductive layer situated on an electrically insulating supporting structure. For a thickness of 0.1 to 1microns, the highly resistive layer has a resistivity of 104 to 105 ohm-cm. The resistive layer limits the currents through the electron-emissive cones so as to protect the field emitter from breakdown and short circuits.
It is desirable to have uniformity of emission from the cathodes. A field emission cathode relies on there being a very strong electric field at the surface of a filament or generally on the surface of the cathode. Creation of the strong field is dependent on, (i) the sharpness of the cathode tip and (ii) the proximity of the extraction electrode (gate) and the cathode. Application of the voltage between these two electrodes produces the strong electric field. Emission nonuniformity is related to the nonuniformity in the relative positions of the emitter tip and the gate. Emission nonuniformity can also result from differences in the sharpness of the emitting tips.
Busta, “Vacuum Microelectronics-1992,” J. Micromech. Microeng., Vol. 2, 1992 pp. 43-74 provides a general review of field-emission devices. Among other things, Busta discusses Utsumi, “Keynote Address, Vacuum Microelectronics: What's New and Exciting,” IEEE Trans. Elect. Dev., Oct. 1990, pp. 2276-2283, who suggests that a filament with a rounded end is the best shape for a field emitter. Also of interest is Fischer et al., “Production and Use of Nuclear Tracks: Imprinting Structure on Solids,” Rev. Mod. Phys., Oct. 1983, pp. 907-948, which deals with the use of charged-particle tracks in manufacturing field emitters according to a replica technique.
A well collimated source of evaporant, as taught in U.S. Pat. No. 3,655,241, is necessary in order to obtain uniformity of cone or filament formation across the entire field emission display. In order to maintain a collimated source, the majority of evaporant is deposited on interior surfaces of the evaporation equipment. The combination of the expensive of the evaporation equipment, and the wastage of evaporant, is undesirable for commercial manufacturing and is compounded as the size of the display increases.
It would be desirable to provide a commercial manufacturing process suitable for large area field emission displays. The commercial viability of this process is due to (i) use of electroplating, in combination with a self alignment method that accommodates for nonuniformities and (ii) the use of spacers as both etch masks and as a part of a mold for plating filament structures.